Parabolic trough power plants use concentrated solar thermal energy to generate electricity by producing steam that drives a Rankine power cycle. Solar thermal energy for steam generation is initially collected in an organic heat transfer fluid (HTF) as it flows through receiver tubes in the solar collector field. The receiver tube is one component of the heat collection element (HCE) shown in FIG. 1. The tube is positioned within a glass tube of larger diameter. Two metal bellows seal the glass and metal tubes at each end to form an annular volume between the outer surface of the metal tube and the inner surface of the glass tube. The annulus is evacuated during normal operation and thermally insulates the metal receiver tube. In effect, the HCE functions as a thermos to minimize heat loss from the HTF as it flows through the receiver tubes in the solar collector field.
The HTF is typically a eutectic mixture of biphenyl and diphenylether, with a maximum operating temperature of about 393° C. and a vapor pressure at this temperature of about 10 atmospheres. At about 393° C., thermal degradation reactions generate hydrogen gas (H2), which can permeate through the walls of the metal tube and occupy the vacuum-filled annular volume of the HCE. Hydrogen gas with low partial pressure has significant thermal conductivity because of its low molecular weight and correspondingly high molecular velocity. The presence of low partial pressures of hydrogen gas in the annulus significantly decreases the thermal performance of the HCE.
Current methods to prevent build-up of hydrogen gas within the annulus include locating a hydrogen getter in the annulus. The getter consists of a material that adsorbs hydrogen to form a hydride and thereby, removes hydrogen from the annular volume. The limitation of this method is the finite capacity of the getter for hydrogen. Once the getter saturates, it cannot adsorb additional hydrogen allowing the concentration of hydrogen in the annulus to increase.
A second method to remove hydrogen from the annulus is to locate a hydrogen permeable membrane as a barrier between the annular volume and ambient air as shown in FIG. 2. The membrane is most commonly a thin layer of palladium that is selectively permeable to hydrogen. At elevated temperatures, hydrogen permeates through the membrane from the annulus to ambient air where it reacts with oxygen to form water. This method works in principle, but practical implementation may result in failure of the palladium membrane or the glass tube when operating at the design temperature.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.